Water flow timing, quantity, and sources in a fractured high mountain permafrost rock wall

Abstract. Water flow in high mountain rock walls is crucial in landscape evolution and slope stability. However, the timing, quantity, and sources of this flow remain poorly understood. In the Mont Blanc massif, tunnels at the Aiguille du Midi peak (3842 m) provide direct access to steep permafrost-affected rock walls. Over two years (May 2022–October 2023), we monitored water flowing from fractures using a real-time system measuring flow rate, temperature, electrical conductivity, and fluorescent tracers, together with meteorological data and ground surface temperatures. Results indicate high surface–subsurface connectivity. The water source is primarily snowmelt, with additional inputs from late-summer rainfall. Electrical conductivity, stable isotopes, and recession curve analysis suggest another source of older subsurface ice. Flow onset was closely tied to ATs, with steady diurnal fluctuations appearing once ground surface temperatures exceeded 0  °C. Lag times between daily peaks of flow rate and peaks of air and ground surface temperatures of 3–9 hours and 0–3 hours, respectively, point to rapid unsaturated infiltration conditions. Distinct flow regimes observed in two adjacent fracture systems reflect a complex, heterogeneous network, including sediment-filled fractures with delayed response. Significant flow rate (often >10 L/h) and water temperature often exceeding 5 °C, suggest a significant heat transfer by advection, capable of enhancing permafrost degradation. This study provides rare direct observations of fracture flow dynamics in steep permafrost rocks, improving understanding of water routing and its response to atmospheric forcing. The findings offer valuable constraints for coupled hydrothermal models, permafrost-related hazard assessments, and the potential impact of climate change.